Photoconductivity measurements used to determine the defect density within the gap of intrinsic semiconductors

J. A. SCHMIDT; C. LONGEAUD; R. R. KOROPECKI; R. ARCE

La Plata

Workshop; 35th Anniversary of Hyperfine Interactions at La Plata - International Workshop; 2005

Universidad Nacional de La Plata

In this work we explore and discus the information that different photoconductivity experiments can provide on the density of states (DOS) within the gap of defective semiconductors. First, we provide an overview of the techniques of modulated photoconductivity (MPC), steady-state photocarrier grating (SSPG) and steady-state photoconductivity (SSPC). In the MPC technique the sample is illuminated by a steady flux of light slightly modulated at a pulsation w. The modulus of the resulting ac current, as well as the phase shift of this current referred to the excitation, are recorded and used to extract information on the DOS. In previous publications we have shown that two regimes have to be considered, the “high frequency” (HF-MPC) and the “low frequency” (LF-MPC) regimes, which provide complementary information on the DOS [1,2]. The basis of the SSPG experiment consists on illuminating the sample with two coherent laser beams of different intensity. If the two beams have the same polarization, a light grating develops between the two electrodes, while if they have perpendicular polarizations the illumination intensity is uniform. As we have shown in a previous publication, by comparing the photocurrents under these two illumination conditions information on the DOS can eventually be extracted [3]. The SSPC is one of the most widely studied properties of semiconductors. A good deal of theoretical work has been devoted to explain the dependence of the photoconductivity upon temperature or light flux, with the aim to derive defect parameters of the material. Experimentally, for different semiconductors a power-law dependence of the photoconductivity on the light flux has been observed, sph µ Gg. In a recent publication we provided an analytical expression for g, and under certain approximations we have shown that a very simple formula relating the DOS at the electron quasi-Fermi level to measurable quantities can be obtained [4]. As an example of the application of these photoconductivity techniques to an experimental situation, in this work we apply these methods to extract the DOS within the gap of a GaAs:Cr crystalline sample. We discuss the interpretation of the experiments with the help of some numerical calculations that simulate the experimental results. Finally, we show that part of the DOS distribution – together with an estimation of the electrons capture cross-sections of these states and of the electrons extended states mobility – can be obtained from the comparison of the results of these techniques. [1] C. Longeaud and J. P. Kleider, Phys. Rev. B 45, 11672 (1992). [2] R.R. Koropecki, J. A. Schmidt, and R. Arce, J. Appl. Phys. 91, 8965 (2002). [3] J. A. Schmidt and C. Longeaud, Phys. Rev. B 71, 125208 (2005). [4] J. A. Schmidt, C. Longeaud, and J. P. Kleider, Thin Solid Films (in press).